In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (“NDE”) methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin et al. (the “'166 Patent”). The '166 Patent discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam must be modulated. Other U.S. patents issued to Monchalin et al. and relating to the subject matter of ultrasonic material testing include the following:
U.S. Pat. No.TitleIssue Date5,608,166Generation and Detection ofMar. 4, 1997Ultrasound with Long PulseLasers4,966,459Broadbank Optical DetectionOct. 30, 1990of Transient Motion from aScattering Surface5,131,748Broadbank Optical DetectionJul. 21, 1992of Transient Motion from aScattering Surface by Two-Wave Mixing in aPhotorefractive Crystal5,402,235Imaging of Ultrasonic-Mar. 29, 1995Surface Motion by OpticalMultiplexing4,633,715Laser HeteroclyneJan. 6, 1987Interferometric Method andApparatus for MeasuringUltrasonic Displacements5,080,491Laser Optical UltrasoundJan. 14, 1992Detection Using TwoInterferometer Apparatuses5,137,361Optical Detection of aAug. 11, 1992Surface Motion of an ObjectUsing a StabilizedInterferometric Cavity4,426,155Method and Apparatus for theJan. 17, 1984Interferometric WavelengthMeasurement of FrequencyTunable C.W. Lasers5,608,166Generation and Detection ofMar. 4, 1991Ultrasound with Long PulseLasers4,820,981Method and Apparatus forApr. 11, 1989Measuring Magnetic Losses inFerromagnetic MaterialsBased on TemperatureModulation Measurements4,659,224Optical InterferometricApr. 21, 1987Reception of UltrasonicEnergy4,607,341Device br DeterminingAug. 19, 1986Properties of Materials froma Measurement of UltrasonicAbsorption
Although these patents describe operable techniques for optically detecting transient motion from a scattering surface, which techniques are useful for ultrasonic composite materials non-destructive test and evaluation, these techniques have numerous failings.
To begin, none of the Monchalin and other known techniques provide the ability to perform with high signal-to-noise-ratios (SNR) at large distances from typically very dark composite materials using small aperture high-speed optical scanning methods. The ability to operate in such a mode has the distinct advantage of increasing the optical scan area coverage and providing substantially improved depth-of-field thereby eliminating the need for active focusing mechanisms.
Other known techniques do not posses the desirable feature of removing common-mode noise from the laser signals using a fully self-referenced interferometric configuration that uses all of the available light without the use of separate stabilization measurements.
Another limitation associated with the Monchalin and other known apparatuses relates to their inability to operate at very high scan rates and process ultrasonic data in real-time. This limitation makes such apparatuses only marginally useful for testing and evaluating composite materials.
Other limitations associated with existing apparatuses relate to general inflexibility of such apparatuses, which may hold all distances low, result in small depth of field performance and only minimal extraction of information from the back scattered signals. These limitations make industrial application of the ultrasonic testing method generally impractical.